Semiconductor light emitting device

ABSTRACT

A semiconductor light emitting device which can suppress the self-absorption of light propagating in a semiconductor film without hindering current spread therein. A reflecting film provided between a support substrate and the semiconductor film of the device includes reflecting electrodes that are in ohmic contact with the semiconductor film and that form current paths between the reflecting electrodes and surface electrodes in the semiconductor film. The reflecting electrodes are in contact with the semiconductor film at such positions that the surface electrodes, provided on the light-extraction-surface-side surface of the semiconductor film, are not over the reflecting electrodes along a direction of the thickness of the semiconductor film. The semiconductor film has reflecting-surface-side recesses made in regions containing regions directly under the surface electrodes and recessed toward the light-extraction-surface side, and reflecting-surface-side protrusions provided in regions containing parts of the semiconductor film in contact with the reflecting electrodes and bonded to the support substrate via the reflecting film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light emitting devicesuch as an LED (light emitting diode) and particularly to a techniquefor improving light extraction efficiency.

2. Description of the Related Art

For LEDs formed of AlGaInP-based material, the band gap of the lightemitting layer is larger than the band gap of a GaAs substrate used incrystal-growth. Hence, part of light directed toward thelight-extraction-surface side out of the light emitted from the lightemitting layer can be extracted, while light directed to theGaAs-substrate side is then absorbed by the GaAs substrate.

Japanese Patent Kokai No. 2002-217450 (Patent Literature 1) discloses anLED which is produced by forming a semiconductor film made ofAlGaInP-based material on a GaAs substrate, then sticking thesemiconductor film to a support substrate via a reflecting film made ofmetal having high reflectance, and then removing the GaAs growthsubstrate. With the LED of this configuration, light directed toward theopposite side from the light extraction surface is reflected by thereflecting film, not absorbed by the GaAs substrate, thus improving thelight extraction efficiency of the LED.

However, light incident at an angle greater than or equal to thecritical angle on the interface between the semiconductor film and anambient medium such as air or resin is totally reflected and cannot beextracted to the outside. Light which could not be extracted outside isrepeatedly reflected inside the semiconductor film (multiplereflection). The intensity of light propagating inside the semiconductorfilm decreases exponentially with propagation distance (optical pathlength). That is, light being multi-reflected inside the semiconductorfilm is absorbed by the semiconductor film (self-absorption) and isdifficult to be extracted outside. For example, if an AlGaInP-basedsemiconductor film having a refractive index of 3.3 is enclosed in resinhaving a refractive index of 1.5, then the critical angle is 27° withthe reflectance at the interface between the semiconductor film and theresin being about 15%, and thus the proportion of light which can beextracted outside is limited to about 4.5%.

Japanese Patent Kokai No. 2008-103627 (Patent Literature 2) discloses asemiconductor light emitting device having recesses/protrusions formedin the light extraction surface of the semiconductor film. With thisconfiguration, light directed to the light extraction surface is thenscattered and diffracted by the recesses/protrusions, thereby reducingthe amount of light totally reflected at the interface between the lightextraction surface and the ambient medium, and thus the light extractionefficiency can be improved. Further, Patent Literature 2 describes thatthe recesses/protrusions are formed without making the thickness of thesemiconductor film smaller. This is because the smaller thickness of thesemiconductor film results in larger series resistance and alsoinsufficient spread of current. That is, insufficient current spread inthe semiconductor film results in the occurrence of a region having highcurrent density. When the current density in the semiconductor filmexceeds a certain level, an overflow of carriers injected into the lightemitting layer occurs, resulting in a decrease in the quantity ofcarriers that can contribute to light emission, thus reducing luminousefficiency. Thus, current needs to be made to widely spread across inthe semiconductor film by securing the thickness of the semiconductorfilm to a certain degree. Like techniques are disclosed in JapanesePatent No. 4230219 (Patent Literature 3) and Japanese Patent Kokai No.2003-258296 (Patent Literature 4).

SUMMARY OF THE INVENTION

Although light is more likely to be extracted outside by forming therecesses/protrusions on the light-extraction-surface side, lightmulti-reflected inside the semiconductor film still exists. As mentionedabove, in order to promote current spread in the semiconductor film, thethickness of the semiconductor film needs to be secured, but the greaterthe thickness of the semiconductor film is, the longer the propagationdistance (optical path length) of light multi-reflected inside thesemiconductor film is, which means that self-absorption is more likelyto occur, thus reducing the light extraction efficiency. That is, it isdifficult to suppress the self-absorption of light propagating insidethe semiconductor film without hindering current spread inside thesemiconductor film.

The present invention has been made in view of the above points, and anobject thereof is to provide a semiconductor light emitting device whichcan suppress the self-absorption of light propagating inside thesemiconductor film without hindering current spread inside thesemiconductor film.

According to the present invention, there is provided a semiconductorlight emitting device which comprises a support substrate; asemiconductor film including a light emitting layer provided on thesupport substrate; surface electrodes provided on a surface on thelight-extraction-surface side of the semiconductor film; and areflecting film forming a reflecting surface and provided between thesupport substrate and the semiconductor film. The reflecting filmincludes reflecting electrodes that are in ohmic contact with thesemiconductor film and that form current paths between the reflectingelectrodes and the surface electrodes in the semiconductor film. Thereflecting electrodes are in contact with the semiconductor film at suchpositions that the surface electrodes are not over the reflectingelectrodes along a direction of the thickness of the semiconductor film.The semiconductor film has, at least at the surface on the side facingthe reflecting surface, reflecting-surface-side recesses made in regionscontaining regions directly under the surface electrodes and recessedtoward the light-extraction-surface side, and reflecting-surface-sideprotrusions provided in regions containing parts of the semiconductorfilm in contact with the reflecting electrodes and bonded to the supportsubstrate via the reflecting film. The reflecting film covers surfacesof the reflecting-surface-side recesses and the reflecting-surface-sideprotrusions.

According to the semiconductor light emitting device of the presentinvention, the self-absorption of light propagating in the semiconductorfilm can be suppressed without hindering current spread therein, thusimproving the light extraction efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the configuration of a semiconductor lightemitting device according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1;

FIG. 3A is a plan view showing the configuration of a terrace structureand surface electrodes on the light-extraction-surface side;

FIG. 3B is a plan view showing the configuration of a terrace structureand reflecting electrodes on the reflecting-surface side;

FIG. 4A is a cross-sectional view showing current paths when counterelectrodes are formed;

FIG. 4B is a cross-sectional view showing the terrace structure that isthe embodiment of the present invention, and current paths;

FIG. 5 is a cross-sectional view showing the configuration of theterrace structure that is the embodiment of the present invention;

FIG. 6 is a cross-sectional view showing the configuration of asemiconductor light emitting device that is another embodiment of thepresent invention;

FIG. 7 is a cross-sectional view showing the configuration of asemiconductor light emitting device that is another embodiment of thepresent invention;

FIGS. 8A to 8C are cross-sectional views showing a manufacturing methodof the semiconductor light emitting device that is the embodiment of thepresent invention;

FIGS. 9A and 9B are cross-sectional views showing the manufacturingmethod of the semiconductor light emitting device that is the embodimentof the present invention;

FIGS. 10A to 10C are cross-sectional views showing the manufacturingmethod of the semiconductor light emitting device that is the embodimentof the present invention;

FIGS. 11A and 11B are SEM micrographs of the semiconductor lightemitting device that is the embodiment of the present invention;

FIG. 12 is a graph showing a relationship between the percentage bywhich a semiconductor film is made thinner and self-absorption reducingeffect;

FIG. 13 is a graph showing current vs. light output characteristics ofthe semiconductor light emitting devices according to the embodiments ofthe present invention; and

FIG. 14 shows saturation currents of the semiconductor light emittingdevices according to the embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described with reference tothe accompanying drawings. In the figures cited below, the samereference numerals are used to denote substantially the same orequivalent constituents or parts.

FIG. 1 is a plan view showing the configuration of a semiconductor lightemitting device 1 according to an embodiment of the present invention,and FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1. Thesemiconductor light emitting device 1 has a so-called stuck-togetherstructure where a semiconductor film 10 and a support substrate 30 arejoined via a reflecting film 20. The semiconductor film 10 is configuredwith an n-type clad layer 11, a light emitting layer 12, a p-type cladlayer 13, and a p-type contact layer 14 that are laid one over anotherin that order from the light-extraction-surface side. The totalthickness of the semiconductor film 10 is, for example, 6 μm, and theoutline of the principal surface is, for example, a square with one sidemeasuring 300 μm. The n-type clad layer 11 is made of, e.g.,(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P, having a layer thickness of 3 μm. Thelight emitting layer 12 has, e.g., a multi-quantum well structure and isconfigured with well layers of about 20 nm thickness made of(Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)P and barrier layers of about 10 nmthickness made of (Al_(0.56)Ga_(0.44))_(0.5)In_(0.5)P that arealternately laid one over the other 15 times. The p-type clad layer 13is made of, e.g., (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P, having a layerthickness of 1 μm. The p-type contact layer 14 is made of, e.g.,Ga_(0.9)In_(0.1)P, having a layer thickness of 1.5 μm. Note that thesemiconductor film 10 is not limited in material to AlGaInP-basedmaterial, but that another material can be used.

The semiconductor film 10 has recesses 60 a, 60 b and protrusions 61 a,61 b that are formed by partially removing both surface areas on thelight-extraction-surface side and the reflecting-surface side oppositethereto, forming a so-called terrace structure. The recess 60 a on thelight-extraction-surface side is recessed toward the reflecting-surfaceside with its bottom being flat. The recess 60 b on thereflecting-surface side is recessed toward the light-extraction-surfaceside with its bottom being flat. Both recesses 60 a and 60 b are formedwithout reaching the light emitting layer 12.

The reflecting film 20 is provided so as to cover the surface of thep-type contact layer 14 in which the recesses 60 b and the protrusions61 b are formed. The reflecting film 20 is constituted by, e.g., adielectric film 22 made of SiO₂ or the like and reflecting electrodes 21made of AuZn or the like. The reflecting electrodes 21 are in contactwith the semiconductor film 10 at the openings of the dielectric film 22to form ohmic contact with the p-type contact layer 14. The reflectingfilm 20 comprising the dielectric film 22 and the reflecting electrodes21 forms a reflecting surface to reflect light emitted from the lightemitting layer 12 toward the light-extraction-surface side at least atthe interface with the semiconductor film 10. The dielectric film 22separates the reflecting electrode 21 into line-like line electrode 21 aand island-like dot electrodes 21 b in the neighborhood of the interfacewith the p-type contact layer 14. The line electrodes 21 a and dotelectrodes 21 b are joined under the dielectric film 22 to beelectrically connected. By configuring the reflecting electrode 21 withthe line electrode 21 a and dot electrodes 21 b in this way, currentconstriction can be prevented so as to make the current density uniformin the semiconductor film 10. The reflecting electrode 21 is formed suchthat the line electrode 21 a and dot electrodes 21 b forming ohmiccontact with the semiconductor film 10 are located on the surface of aprotrusion 61 b on the reflecting-surface side. No line electrode 21 aand dot electrode 21 b are formed on the recesses 60 b on thereflecting-surface side. Further, the line electrode 21 a and dotelectrodes 21 b are provided at such positions that an ohmic electrode43 on the light-extraction-surface side is not over them along adirection of the semiconductor film thickness. That is, the reflectingelectrode 21 is in contact with the semiconductor film 10 at such aposition that the surface electrode (ohmic electrode 43) is not over italong a direction of the semiconductor film 10 thickness, forming aso-called counter electrode. Note that instead of SiO₂, anothertransparent dielectric material such as Si₃N₄ or Al₂O₃ can be used forthe dielectric film 22, and that not being limited to AuZn, anothermaterial having high light reflectivity which can form ohmic contactwith the p-type contact layer 14 is preferably used for the reflectingelectrodes 21.

In the present embodiment, with the reflecting electrodes 21 being madeof AuZn and the dielectric film 22 being made of SiO₂ (with a coveringrate of about 85%), the reflectance on the reflecting-surface side isabout 94%. Higher reflectance on the reflecting-surface side is morepreferable, and where a light extraction structure is provided on thelight-extraction-surface side, it is preferable to make regularreflectance high. Where a light extraction structure is not provided, itis preferable to make diffuse reflectance high.

A barrier metal layer 26 and an adhesive layer 27 are provided on thereflecting film 20. The barrier metal layer 26 can be constituted by asingle layer or two or more layers including high melting-point metalsuch as Ta, Ti, or W, or a nitride thereof. The barrier metal layer 26prevents Zn included in the reflecting electrodes 21 from diffusingoutside and prevents eutectic binding material (e.g., AuSn) included inthe junction layer 33 from diffusing into the reflecting electrodes 21.The adhesive layer 27 is constituted by a laminated film of, e.g., Niand Au and has a function to improve wettability to the eutectic bindingmaterial included in the junction layer 33. Thereby, the supportsubstrate 30 and the semiconductor film 10 are bonded well.

A Schottky electrode 41 and the ohmic electrodes 43 constituting surfaceelectrodes are formed on the surface of the n-type clad layer 11 that isthe light extraction surface. The Schottky electrode 41 constitutes abonding pad and can be made of material which can form Schottky contactwith the n-type clad layer 11 such as Ta, Ti, W, or an alloy thereof.Alternatively, it can be made of an insulating dielectric such as SiO₂instead of metal material. An Au layer may be formed on the outermostsurface of the Schottky electrode 41 to improve the wire-bonding-abilityand conductivity thereof. The Schottky electrode 41 is formed on thesurface (bottom of the recess 60 a) of an area in which is formed arecess 60 a on the light-extraction-surface side of the semiconductorfilm 10 having the terrace structure. The ohmic electrodes 43 are madeof material which can form ohmic contact with the n-type clad layer 11such as AuGeNi, AuSn, or AuSnNi and are formed on the surface of theprotrusions 61 a on the light-extraction-surface side of thesemiconductor film 10. The Schottky electrode 41 and the ohmicelectrodes 43 are electrically connected through connection lines 42joining both electrodes. The connection lines 42 are made of the samematerial as the Schottky electrode 41 and form Schottky contact with then-type clad layer 11. Because the Schottky electrode 41 forms Schottkycontact with the n-type clad layer 11, current does not flow throughpart of the semiconductor film 10 directly under the Schottky electrode41. Further, the reflecting electrode 21 on the reflecting-surface sideand the ohmic electrode 43 on the light-extraction-surface side areformed at such positions that one is not over the other along adirection of the semiconductor film 10 thickness. The recesses 60 a onthe light-extraction-surface side are formed over the reflectingelectrodes 21, and hollows forming the recesses 60 b on thereflecting-surface side are formed directly under the ohmic electrodes43. That is, current flows between the ohmic electrode 43 on thelight-extraction-surface side and the reflecting electrode 21 on thereflecting-surface side. In FIG. 2, paths of current flowing through thesemiconductor film 10 are indicated by arrows.

The support substrate 30 is a Si substrate given conductivity by dopingwith, e.g., a p-type impurity at a high concentration. Ohmic metallayers 31 and 32 of, e.g., Pt are formed on opposite surfaces of thesupport substrate 30, which is bonded to the reflecting film 20 via thejunction layer 33. The junction layer 33 has a laminated structurewhere, e.g., Ti, Ni, and AuSn are formed in that order from the sideclose to the support substrate 30. Note that instead of Si, anotherconductive material such as Ge, Al, or Cu can be used for the supportsubstrate 30.

In FIG. 1, the Schottky electrode 41 and the ohmic electrodes 43constituting surface electrodes on the light-extraction-surface side areshown on the same plane as the line electrodes 21 a and dot electrodes21 b forming the reflecting electrodes 21 on the reflecting-surfaceside. Line electrodes 21 a and dot electrodes 21 b on thereflecting-surface side are located on opposite sides of, and along,each of eight electrode pieces constituting the ohmic electrodes 43 onthe light-extraction-surface side. In other words, the line electrodes21 a on the reflecting-surface side are formed to surround the electrodepieces constituting the ohmic electrodes 43 on thelight-extraction-surface side, with each of the electrode pieces beingplaced in the middle of an area enclosed by line electrodes 21 a on thereflecting-surface side. The ohmic electrode 43 on thelight-extraction-surface side and the line electrode 21 a and dotelectrodes 21 b on the reflecting-surface side are placed such that oneis not over the other along a direction of the semiconductor film 10thickness, forming so-called counter electrodes. Because ofconfiguration with counter electrodes, current can be widely spread inthe semiconductor film 10 even with the area of the ohmic electrodes 43on the light-extraction-surface side being smaller. Thus, the coveringrate of the electrodes in the light extraction surface can be reduced,thereby improving the light extraction efficiency. Further, with theconfiguration with counter electrodes, the recesses 60 a on thelight-extraction-surface side and the recesses 60 b on thereflecting-surface side can be enlarged in area because of arelationship with current paths described later.

FIG. 3A shows the configuration of the recesses 60 a, protrusions 61 a,ohmic electrodes 43, Schottky electrode 41, and connection lines 42formed on the surface on the light-extraction-surface side of thesemiconductor film 10. FIG. 3B shows the configuration of the recesses60 b, protrusions 61 b, and reflecting electrodes 21 formed on thesurface on the reflecting-surface side of the semiconductor film 10. Inthe present embodiment, the outlines of the recess 60 a on thelight-extraction-surface side and the protrusion 61 b on thereflecting-surface side are of the same shape, and these outlines arelocated one over the other along a direction of the semiconductor film10 thickness. Likewise, the outlines of the protrusion 61 a on thelight-extraction-surface side and the recess 60 b on thereflecting-surface side are of the same shape, and these outlines arelocated one over the other along a direction of the semiconductor film10 thickness.

(Relationship Between the Terrace Structure and Current Paths)

As described above, the semiconductor film 10 has the recesses 60 a, 60b and the protrusions 61 a, 61 b formed by partially removing bothsurface areas on the light-extraction-surface side and on thereflecting-surface side opposite thereto. That is, spaces left after thesemiconductor film 10 has been partially removed correspond to therecesses 60 a, 60 b, and other parts than have been removed correspondto the protrusions 61 a, 61 b.

Two reflecting electrodes 21 each comprising the line electrode 21 a anddot electrodes 21 b are placed on opposite sides of the ohmic electrode43 on the light-extraction-surface side, and current flows, spreadingleft and right, from the ohmic electrode 43 on thelight-extraction-surface side to the reflecting electrodes 21 in thesemiconductor film 10 as shown in FIG. 4A. Because of the configurationwith these counter electrodes, regions left out of the current paths,that is, regions not contributing to current spread (indicated byhatching in FIG. 4A) occur in both surface areas on thelight-extraction-surface side and on the reflecting-surface side of thesemiconductor film 10. Note that in FIG. 4A the semiconductor film 10having no terrace structure is shown for the sake of description. Asshown in FIG. 4B, in the semiconductor light emitting device 1 accordingto the present embodiment, by removing parts not contributing to thecurrent spread in the semiconductor film 10 shown in FIG. 4A, therecesses 60 a on the light-extraction-surface side and the recesses 60 bon the reflecting-surface side are formed. That is, the recesses 60 a,60 b are provided at such positions that they do not cross the currentpaths formed between the ohmic electrode 43 on thelight-extraction-surface side and the reflecting electrodes 21. At areaswhere part of the semiconductor film 10 is removed, the distance(optical path length) between the reflecting surface and the lightextraction surface is shorter. Thus, the self-absorption of light beingmulti-reflected in the semiconductor film 10 can be suppressed, therebyimproving the light extraction efficiency. Because the regions wherematerial of the semiconductor film 10 is removed are out of the currentpaths, the current spread in the semiconductor film 10 is not hindered.As such, the semiconductor film 10 is partially made thinner by removingparts not contributing to the current spread in the semiconductor film10, and thereby the self-absorption of light propagating in thesemiconductor film 10 can be suppressed without hindering the currentspread.

(Configuration of the Terrace Structure on the Light-Extraction-SurfaceSide)

The light-extraction-surface-side ohmic electrode 43 is formed on thesurface of the protrusion 61 a on the light-extraction-surface side ofthe semiconductor film 10. The recess 60 a on thelight-extraction-surface side is provided in a region containing aregion directly above the contact between the reflecting electrode 21and the semiconductor film 10. The proportion (surface ratio) occupiedby the recesses 60 a of the surface on the light-extraction-surface sideof the semiconductor film 10 is preferably 15% or greater. If the areaof the recesses 60 a is made too small, the effect of improving thelight extraction efficiency will be reduced. On the other hand, if thearea of the recesses 60 a is made too large, the recesses 60 a will cutoff the current paths, thereby hindering the current spread. As shown inFIG. 5, the distance Wu from the light-extraction-surface-side ohmicelectrode 43 to an edge of the recess 60 a is set at 30 to 70%,preferably 40 to 60%, of the horizontal distance L from the ohmicelectrode 43 on the light-extraction-surface side to the reflectingelectrode 21 on the reflecting-surface side. Note that the horizontaldistance refers to a distance measured when the images of the ohmicelectrode 43 on the light-extraction-surface side and the reflectingelectrode 21 on the reflecting-surface side are projected onto the sameplane parallel to the principal surface of the semiconductor film 10.The depth Hu of the recess 60 a on the light-extraction-surface side ispreferably at 15% or greater of the total thickness D of thesemiconductor film 10 and at 25 to 75% of the thickness dn of the n-typeclad layer 11. Note that where the semiconductor film 10 includes alayer of n-type conductivity as well as the n-type clad layer 11, the dnrefers to the sum of the thicknesses of all the layers of n-typeconductivity. Although the region where the recess 60 a is formed is aregion that does not contribute to the current spread, if the n-typeclad layer 11 is removed partially down to such a depth as to reach thelight emitting layer 12, at the material-removed region, carriers cannotbe injected into the light emitting layer 12, resulting in theoccurrence of a non-light emitting region. Hence, the n-type clad layer11 is preferably not completely removed.

(Configuration of the Terrace Structure on the Reflecting-Surface Side)

The reflecting electrode 21 is formed on the surface of the protrusion61 b on the reflecting-surface side of the semiconductor film 10. Theprotrusions 61 b are bonded to the support substrate 30 via thereflecting film 20. The recess 60 b on the reflecting-surface side isformed in a region containing a region directly under thelight-extraction-surface-side ohmic electrode 43. The bottom of therecess 60 b is separated by a space 70 from the support substrate 30.

The Schottky electrode 41 on the light-extraction-surface side isprovided over the protrusion 61 b on the reflecting-surface side, andthe recess 60 b on the reflecting-surface side is formed so as not to belocated directly under the Schottky electrode 41. This is because withthe Schottky electrode 41 forming a bonding pad, if the recess 60 b onthe reflecting-surface side existed directly under the bonding pad, thesemiconductor film 10 might be damaged by the pressing force of abonding tool. The proportion (surface ratio) occupied by the recesses 60b of the surface on the reflecting-surface side of the semiconductorfilm 10 is preferably 15% or greater but not greater than 50%. If thearea of the recesses 60 b is made too small, the effect of improving theluminous efficiency will be reduced. On the other hand, if the area ofthe recesses 60 b is made too large, the current spread is hindered andalso the mechanical strength of the semiconductor film 10 is reduced,resulting in the occurrence of a problem in reliability. As shown inFIG. 5, the distance W1 from the reflecting electrode 21 to an edge ofthe recess 60 b is set at 30 to 70%, preferably 40 to 60%, of thehorizontal distance L from the ohmic electrode 43 on thelight-extraction-surface side to the reflecting electrode 21 on thereflecting-surface side. The depth H1 of the recess 60 b on thereflecting-surface side is preferably at 15% or greater of the totalthickness D of the semiconductor film 10 and at 25 to 75% of the totalthickness dp of the p-type contact layer 14 and the p-type clad layer13. Note that where the semiconductor film 10 includes a layer of p-typeconductivity as well as the p-type contact layer 14 and the p-type cladlayer 13, the dp refers to the sum of the thicknesses of all the layersof p-type conductivity. Although the region where the recess 60 b isformed is a region that does not contribute to the current spread, ifthe p-type contact layer 14 and the p-type clad layer 13 are removedpartially down to such a depth as to reach the light emitting layer 12,at the material-removed region, carriers cannot be injected into thelight emitting layer 12, resulting in the occurrence of a non-lightemitting region. Hence, the p-type clad layer 13 is preferably notcompletely removed.

Although the above embodiment is configured such that the outline of therecess 60 a on the light-extraction-surface side and the outline of theprotrusion 61 b on the reflecting-surface side are located one over theother along a direction of the semiconductor film 10 thickness and thatthe outline of the protrusion 61 a on the light-extraction-surface sideand the outline of the recess 60 b on the reflecting-surface side arelocated one over the other (the recess 60 a is not over the recess 60 band the protrusion 61 a is not over the protrusion 61 b), the presentinvention is not limited to this. That is, it does not matter whether ornot the recess 60 a on the light-extraction-surface side and the recess60 b on the reflecting-surface side are located one over the other alonga direction of the semiconductor film 10 thickness and whether or notthe protrusion 61 a on the light-extraction-surface side and theprotrusion 61 b on the reflecting-surface side are located one over theother. Further, in the above embodiment, both surfaces on thelight-extraction-surface side and on the reflecting-surface side of thesemiconductor film 10 are made to have the terrace structure, but theterrace structure needs to be provided at least on thereflecting-surface side. This is because, while recesses can be formedon either side to produce the effect of suppressing the self-absorption,for light which is reflected on the reflecting-surface side and sent outthrough the light extraction surface out of the light emitted from thelight emitting layer, the self-absorption suppressing effect associatedwith the optical length shortening effect produced by the terracestructure formed on the reflecting-surface side is extremely large. FIG.6 is a cross-sectional view of the semiconductor light emitting device 1having the terrace structure only on the reflecting-surface side of thesemiconductor film 10. It is possible to suppress the self-absorption oflight propagating in the semiconductor film without hindering thecurrent spread by making either surface of the semiconductor film 10have the terrace structure. Although in the above embodiment the p-sideof the semiconductor film 10 is the reflecting surface with the n-sidethereof being the light extraction surface, the n-side may be thereflecting surface with the p-side being the light extraction surface.

(Adding a Light Extraction Structure)

As shown in FIG. 7, in the semiconductor light emitting device 1according to the present embodiment, the light extraction efficiency canbe further improved by adding a light extraction structure 80 to thesurface on the light-extraction-surface side of the semiconductor film10. Specifically, the surface of the semiconductor film is coarsened, ora photonic crystal comprising multiple protrusions or holes is formed inthe surface of the semiconductor film. It is possible to suppress theself-absorption of light propagating in the semiconductor film 10 bymaking the semiconductor film 10 have the terrace structure. However,light multi-reflection itself cannot be effectively prevented only bymaking the semiconductor film 10 have the terrace structure. By formingfine recesses/protrusions in the light extraction surface of thesemiconductor film 10, the amount of light totally reflected at theinterface between the semiconductor film 10 and the ambient medium canbe reduced. That is, by combining the terrace structure and the lightextraction structure, light can be extracted outside with maintaininghigh light output because of synergistic action of the self-absorptionsuppressing effect and the multi-reflection suppressing effect. Withthis configuration, the light extraction efficiency can be furtherimproved as compared with a conventionally-structured semiconductorlight emitting device having only the light extraction structure.

Next, a manufacturing method of the semiconductor light emitting device1 according to the embodiment of the present invention will bedescribed. In the description below, there is shown a manufacturingmethod of a semiconductor light emitting device which has the terracestructure in opposite surfaces of the semiconductor film and has thelight extraction structure in the surface on thelight-extraction-surface side.

(Semiconductor Film Forming Process)

The semiconductor film 10 was formed by a metal organic chemical vapordeposition method (MOCVD method). An n-type GaAs substrate of 300 μmthickness which slopes at an angle of 15° in the [011] directionrelative to the (100) plane was used as a growth substrate 50 for use inthe crystal growth of the semiconductor film 10. The n-type clad layer11 made of (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P and 3 μm thick was formedon the growth substrate 50. The light emitting layer 12 was formed onthe n-type clad layer 11. The light emitting layer 12 was made to have amulti-quantum well structure where well layers made of(Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)P, 20 nm thick and barrier layers madeof (Al_(0.56)Ga_(0.44))_(0.5)In_(0.5)P, 10 nm thick are alternately laidone over the other 15 times. Note that the proportion of Al of the welllayers can be changed in the range of 0≦z≦0.4, corresponding to thelight emission wavelength. The p-type clad layer 13 made of(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P and 1 μm thick was formed on the lightemitting layer 12. Note that the proportion z of Al of the n-type cladlayer 11 and the p-type clad layer 13 can be changed in the range of0.4≦z≦1.0. The p-type contact layer 14 made of Ga_(0.90.5)In_(0.4)P and1.5 μm thick was formed on the p-type clad layer 13. The proportion ofIn of the p-type contact layer 14 can be changed in such a range thatthe layer does not absorb light from the light emitting layer 12. Thesemiconductor film 10 of 6 μm thickness is composed of these layers(FIG. 8A). Note that phosphine (PH₃) was used as a Group V material andthat organic metals which are trimethylgallium (TMGa), trimethylaluminum(TMAl), and trimethylindium (TMI) were used as Group III materials.Silane (SiH₄) was used as material for Si that is an n-type impurity,and dimethylzinc (DMZn) was used as material for Zn that is a p-typeimpurity. Growth temperature was 750 to 850° C.; hydrogen was used as acarrier gas; and growth pressure was 10 kPa.

(Forming Process of the Terrace Structure on the Reflecting-SurfaceSide)

By processing the p-type contact layer 14 on the reflecting-surfaceside, the terrace structure was formed on the reflecting-surface side ofthe semiconductor film 10. A mask of SiO₂ was formed on the p-typecontact layer 14, and parts of the p-type contact layer 14 exposedthrough openings of the mask were removed by dry etching to form therecesses 60 b. The protrusions 61 b are formed in association with theformation of the recesses 60 b. By controlling the etching time, thedepth of the recesses 60 b was set at 1.5 μm, which corresponds to 60%of the thickness of the p-type semiconductor layer including the p-typecontact layer 14 and the p-type clad layer 13 and 25% of the totalthickness of the semiconductor film 10. The proportion (area ratio)occupied by the recesses 60 b of the surface on the reflecting-surfaceside of the semiconductor film 10 was set at 30% (FIG. 8B). The bottomof the recesses 60 b may reach the p-type clad layer 13, but the etchingshould not be deep down to the light emitting layer 12. Instead, wetetching can be used as the etching method.

(Forming Process of the Reflecting Film and Metal Layer)

An SiO₂ film constituting the dielectric film 22 was formed by a plasmaCVD method on the p-type contact layer 14 to cover the surfaces of therecesses 60 b and the protrusions 61 b. The thickness t of the SiO₂ filmis set to satisfy t=m·λ_(C)/4n, where λ₀ is the light emissionwavelength in a vacuum, n is the refractive index of the SiO₂ film, andm is an arbitrary integer. Here, letting λ₀=625 nm, n=1.45, and m=3,then the thickness t of the dielectric film 22 was 320 nm. Subsequently,a resist mask was formed on the SiO₂ film, and then etching was performusing buffered hydrofluoric acid (BHF) to perform patterningcorresponding to the reflecting electrodes on the SiO₂ film. Openingswere formed in etched-away regions of the SiO₂ film, and through theopenings, the p-type contact layer 14 is partially exposed. Note that athermal CVD method or a sputtering method can also be used as the filmforming method of the SiO₂ film. Further, a dry etching method can beused as the etching method of the SiO₂ film. Instead of SiO₂, anothertransparent dielectric material such as Si₃N₄ or Al₂O₂ can be used forthe dielectric film 22.

Then, the reflecting electrodes 21 made of AuZn and 300 nm thick wereformed on the dielectric film 22 by an EB evaporation method. Thereflecting electrodes 21 are in contact with the p-type contact layer 14through the openings formed in the dielectric film 22 by the previousetching. The reflecting electrode 21 is separated by the dielectric film22 into the line-like line electrode 21 a and island-like dot electrodes21 b. The reflecting film 20 is comprised of the dielectric film 22 andthe reflecting electrodes 21.

Next, TaN (100 nm), TiW (100 nm), and TaN (100 nm) were sequentiallydeposited over the reflecting film 20 by a sputtering method to form thebarrier metal layer 26. Note that the barrier metal layer 26 may beconstituted by a single layer or two or more layers including highmelting-point metal such as Ta, Ti, or W, or a nitride thereof. Thebarrier metal layer 26 may be formed using an EB evaporation methodinstead of the sputtering method. Then, heat treatment was performed ina nitrogen atmosphere at about 500° C. Thereby, good ohmic contact wasformed between the reflecting electrodes 21 and the p-type contact layer14.

Then, Ni (300 nm) and Au (30 nm) were sequentially formed over thebarrier metal layer 26 by an EB evaporation method to form the adhesivelayer 27. Note that a resistance heating evaporation method or asputtering method can be used to form the adhesive layer 27 (FIG. 8C).

(Support Substrate Bonding Process)

A Si substrate given conductivity by doping with a p-type impurity wasused as the support substrate 30 for supporting the semiconductor film10. The ohmic metal layers 31 and 32 made of Pt and 200 nm thick wereformed on opposite surfaces of the support substrate 30. Subsequently,Ti (150 nm), Ni (100 nm), and AuSn (600 nm) were sequentially depositedover the ohmic metal layer 32 by a sputtering method to form thejunction layer 33. The AuSn layer is used as the eutectic bindingmaterial, and its composition desirably consists of 70 to 80 wt % of Auand 20 to 30 wt % of Sn. The Ni layer has a function to improvewettability to the eutectic binding material. Instead of Ni, NiV or Ptcan be used. The Ti layer has a function to improve adhesion between theNi and the ohmic metal layer 32. For the ohmic metal layers 31, 32,another material which can form ohmic contact with the Si substrate suchas Au, Ni, or Ti can be used, not being limited to Pt. The supportsubstrate 30 may be made of another material which has conductivity andhigh thermal conductivity such as Ge, Al, or Cu.

The semiconductor film 10 and the support substrate 30 were bonded bythermal compression. The adhesive layer 27 on the semiconductor film 10side and the junction layer 33 on the support substrate 30 side were putin close contact and kept in a nitrogen atmosphere at 1 MPa and 330° C.for 10 minutes. The eutectic binding material (AuSn) included in thejunction layer 33 on the support substrate 30 side melts and, togetherwith the adhesive layer 27 (Ni/Au) on the semiconductor film 10 side,forms AuSnNi and thereby the support substrate 30 and the semiconductorfilm 10 are bonded together (FIG. 9A).

(Growth Substrate Removing Process)

The growth substrate 50 used in the crystal growth of the semiconductorfilm 10 was removed by wet etching using a mixture of ammonia water andhydrogen peroxide water. Note that a dry etching method, a mechanicalpolishing method, or a chemical mechanical polishing method (CMP) may beused for removing the growth substrate 50 (FIG. 9B).

(Forming Process of the Terrace Structure on theLight-Extraction-Surface Side)

By processing the n-type clad layer 11 exposed by removing the growthsubstrate 50, the terrace structure was formed on thelight-extraction-surface side of the semiconductor film 10. A mask ofSiO₂ was formed on the n-type clad layer 11, and parts of the n-typeclad layer 11 exposed through openings of the mask were removed by dryetching to form the recesses 60 a on the light-extraction-surface side.The protrusions 61 a on the light-extraction-surface side are formed inassociation with the formation of the recesses 60 a. By controlling theetching time, the depth of the recesses 60 a was set at 1.5 μm, whichcorresponds to 50% of the thickness of the n-type clad layer 11 and 25%of the total thickness of the semiconductor film 10. The proportion(area ratio) occupied by the recesses 60 a of the surface on thelight-extraction-surface side of the semiconductor film 10 was set at70% (FIG. 10A). Instead, wet etching can be used as the etching method.

(Light Extraction Structure Forming Process)

By finely processing the surface of the n-type clad layer 11, thephotonic crystal 80 was formed to improve the light extractionefficiency. A mask for an artificial periodic structure was formed onthe n-type clad layer 11 by photolithography and a lift-off method, andthen multiple conic projections, in a triangle lattice array, of aperiod of 500 nm, a height of 600 nm, and an aspect ratio of 1.2 wereformed in the surface of the n-type clad layer 11 (FIG. 10B). Instead, afine processing technology such as EB lithography or nano-imprint can beused in forming the mask pattern. Not being limited to conicprojections, the photonic crystal may be in the form of cylindricalprojections or pyramid projections, or may comprise multiple holes orgrooves. Further, by coarsening the surface of the n-type clad layer 11by wet etching, the light extraction structure may be formed. The aboveprocessing may be performed after a mask is provided on theelectrode-to-be-formed regions on the light-extraction-surface side asneeded. The light extraction structure may be provided on the slopingsurfaces of the protrusions 61 a.

(Forming Process of the Electrodes on the Light-Extraction-Surface Side)

The ohmic electrodes 43, Schottky electrode 41, and connection lines 42were formed on the n-type clad layer 11. After AuGeNi was deposited byan EB evaporation method over the n-type clad layer 11 to form ohmiccontact with the n-type clad layer 11, patterning was performed by alift-off method to form the ohmic electrodes 43. Subsequently, Ti (100nm) was deposited by an EB evaporation method over the n-type clad layer11 to form Schottky contact with the n-type clad layer 11, and furtherAu (1.5 μm) was deposited over the Ti. Then, patterning was performed bya lift-off method to form the Schottky electrode 41 and connection lines42. Note that AuGe, AuSn, AuSnNi, or the like can also be used for theohmic electrodes 43. Further, Ta, W, or an alloy thereof, or a nitridethereof can also be used for the Schottky electrode 41. Then, heattreatment was performed in a nitrogen atmosphere at 400° C. to promotethe formation of ohmic contact between the n-type clad layer 11 and theohmic electrodes 43 (FIG. 10C). By undergoing the above processes, thesemiconductor light emitting device 1 is finished.

FIGS. 11A and 11B are SEM micrographs of the semiconductor lightemitting device produced through the above production process; FIG. 11Ashows an image of the light extraction surface of the semiconductor film10 in which the photonic crystal is formed; and FIG. 11B shows an imageof a cross-section of the semiconductor light emitting device.

(Evaluation Results)

First, the light self-absorption suppressing effect when thesemiconductor film is made thinner was examined. FIG. 12 shows theresults of estimating the effect. In FIG. 12, the horizontal axisrepresents the percentage by which the semiconductor film 10 was madethinner, and the vertical axis represents the self-absorptionsuppressing effect for light propagating in the semiconductor film. Theself-absorbed light amount increases exponentially with the propagationdistance (optical path length) of light in the semiconductor film.Hence, the light output increases at a greater rate than the thicknessof the semiconductor film changes. For example, it is expected thatreducing the thickness of the semiconductor film by 50% will result in areduction of about 65% in light self-absorption.

Next, a current vs. light output characteristic of the semiconductorlight emitting device 1 according to the embodiment of the presentinvention was obtained. Assessed samples were of the following fourtypes: one with opposite surfaces of the semiconductor film having theterrace structure (sample 1), one with opposite surfaces of thesemiconductor film having the terrace structure and further the photoniccrystal formed in the light extraction surface (sample 2), one as acomparative example without the terrace structure in the semiconductorfilm and with the photonic crystal formed (sample 3), and one as anothercomparative example with neither the terrace structure nor the photoniccrystal formed (sample 4). FIG. 13 is a graph showing current vs. lightoutput characteristics of the assessed samples 1 to 4. Taking the onewith neither the terrace structure nor the photonic crystal formed(sample 4) as a reference, the light output at 90 mA was improved by 14%for the one with opposite surfaces of the semiconductor film having theterrace structure (sample 1). For the one without the terrace structureformed and with the photonic crystal formed (sample 3), the light outputat 90 mA was improved by 29%. For the one with opposite surfaces of thesemiconductor film having the terrace structure formed and further thephotonic crystal formed on the light extraction surface (sample 2), thelight output at 90 mA was improved by 52%. When comparing the samples 1and 4, and the samples 2 and 3, in either case, the light output of theone with the semiconductor film having the terrace structure wasimproved. From the above results, it was confirmed that by making thesemiconductor film have the terrace structure, the light extractionefficiency can be improved.

Then, in order to confirm whether the current spread was hindered bymaking the semiconductor film have the terrace structure, the currentvalue when the light output is saturated (called a saturation currentvalue) was measured for each of the above samples 1 to 4. That is, ifthe current spread is hindered, current constriction occurs, making thedensity of carriers injected into the light emitting layer become higherwhile the current value is the same, resulting in a decrease in thesaturation current. FIG. 14 shows results of measuring the saturationcurrent of the above samples 1 to 4, and the vertical axis indicatesvalues normalized to the saturation current value of the one withneither the terrace structure nor the photonic crystal formed in thesemiconductor film (sample 4). There is seen a difference in saturationcurrent value between the ones with the photonic crystal formed (samples2, 3) and the ones without the photonic crystal (samples 1, 4), whereasthere is not seen a difference in saturation current value between theones with the terrace structure formed (samples 1, 2) and the oneswithout the terrace structure (samples 3, 4). This means that if theterrace structure is formed in such a way as not to cut off the currentpaths, the current spread is not hindered and that thus the luminousefficiency can be maintained.

Next, the light outputs of the ones with the semiconductor film havingthe terrace structure and of the ones without having the terracestructure were compared. The light output was improved by 13% for theone without the photonic crystal formed and with the semiconductor filmhaving the terrace structure. Meanwhile, for the one with the photoniccrystal formed and with the semiconductor film having the terracestructure, the light output was improved by 18%. That is, when the lightextraction structure and the terrace structure are combined, the effectof improving the light extraction efficiency becomes more conspicuous.This is because of synergistic action of the multi-reflectionsuppressing effect of the light extraction structure and theself-absorption suppressing effect of the terrace structure.

As apparent from above description, according to the semiconductor lightemitting device of the present invention, parts of the semiconductorfilm that do not contribute to the current spread are removed, therebymaking the semiconductor film partially thinner. Hence, the propagationdistance (optical path length) of light multi-reflected inside thesemiconductor film can be shortened without hindering the currentspread. By this means, without current constriction occurring, theself-absorption of light can be suppressed, thus improving the lightextraction efficiency.

This application is based on Japanese Patent Application No. 2010-033940which is herein incorporated by reference.

1. A semiconductor light emitting device which comprises a supportsubstrate; a semiconductor film including a light emitting layerprovided on said support substrate; surface electrodes provided on asurface on the light-extraction-surface side of said semiconductor film;and a reflecting film forming a reflecting surface and provided betweensaid support substrate and said semiconductor film, wherein saidreflecting film includes reflecting electrodes that are in ohmic contactwith said semiconductor film and that form current paths between thereflecting electrodes and said surface electrodes in said semiconductorfilm, wherein said reflecting electrodes are in contact with saidsemiconductor film at such positions that said surface electrodes arenot over the reflecting electrodes along a direction of the thickness ofsaid semiconductor film, wherein said semiconductor film has, at leastat the surface on the side facing said reflecting surface,reflecting-surface-side recesses made in regions containing regionsdirectly under said surface electrodes and recessed toward saidlight-extraction-surface side, and reflecting-surface-side protrusionsprovided in regions containing parts of said semiconductor film incontact with said reflecting electrodes and bonded to said supportsubstrate via said reflecting film, and wherein said reflecting filmcovers surfaces of said reflecting-surface-side recesses and saidreflecting-surface-side protrusions.
 2. A semiconductor light emittingdevice according to claim 1, wherein said reflecting-surface-siderecesses do not cross said current paths.
 3. A semiconductor lightemitting device according to claim 1, wherein said semiconductor filmincludes a p-type semiconductor layer and an n-type semiconductor layerbetween which said light emitting layer is sandwiched, wherein saidreflecting-surface-side recesses are formed by partially removing saidp-type semiconductor layer or said n-type semiconductor layer, andwherein the depth of said reflecting-surface-side recesses is at 25% orgreater but not greater than 75% of the thickness of the p-typesemiconductor layer or the n-type semiconductor layer in which saidreflecting-surface-side recesses are formed and at 15% or greater of thetotal thickness of said semiconductor film.
 4. A semiconductor lightemitting device according to claim 1, wherein of the surface on saidreflecting-surface side of said semiconductor film, the proportionoccupied by regions where said reflecting-surface-side recesses areformed is 15% or greater but not greater than 50%.
 5. A semiconductorlight emitting device according to claim 1, wherein said surfaceelectrodes include ohmic electrodes that form ohmic contact with saidsemiconductor film, and wherein said semiconductor film further haslight-extraction-surface-side recesses made in other regions than parts,in contact with said ohmic electrodes, of the surface on saidlight-extraction-surface side thereof and recessed toward saidreflecting-surface side.
 6. A semiconductor light emitting deviceaccording to claim 5, wherein said light-extraction-surface-siderecesses do not cross said current paths.
 7. A semiconductor lightemitting device according to claim 5, wherein said semiconductor filmincludes a p-type semiconductor layer and an n-type semiconductor layerbetween which said light emitting layer is sandwiched, wherein saidlight-extraction-surface-side recesses are formed by partially removingsaid p-type semiconductor layer or said n-type semiconductor layer, andwherein the depth of said light-extraction-surface-side recesses is at25% or greater but not greater than 75% of the thickness of the p-typesemiconductor layer or the n-type semiconductor layer in which saidlight-extraction-surface-side recesses are formed and at 15% or greaterof the total thickness of said semiconductor film.
 8. A semiconductorlight emitting device according to any of claims 5, wherein saidlight-extraction-surface-side recesses are provided in regionscontaining regions directly above contacts between said reflectingelectrodes and said semiconductor film.
 9. A semiconductor lightemitting device according to claim 5, wherein of the surface on saidlight-extraction-surface side of said semiconductor film, the proportionoccupied by regions where said light-extraction-surface-side recessesare formed is 15% or greater.
 10. A semiconductor light emitting deviceaccording to any of claim 5, wherein said surface electrodes furtherinclude a Schottky electrode that forms Schottky contact with saidsemiconductor film, and wherein said Schottky electrode is provided overone of said reflecting-surface-side protrusions.
 11. A semiconductorlight emitting device according to any of claims 5, wherein the outlinesof said light-extraction-surface-side recesses are over the outlines ofsaid reflecting-surface-side protrusions respectively along a directionof the thickness of said semiconductor film.
 12. A semiconductor lightemitting device according to claim 1, wherein said semiconductor filmfurther has multiple projections or holes or grooves in its surface onsaid light-extraction-surface side to extract light emitted from saidlight emitting layer outside.
 13. A semiconductor light emitting deviceaccording to claim 5, wherein said semiconductor film further hasmultiple projections or holes or grooves in its surface on saidlight-extraction-surface side to extract light emitted from said lightemitting layer outside.